The increasing worldwide incidence of diabetes mellitus in adults constitutes a global public health burden. It is predicted that by 2030, India, China and the United States will have the largest number of people with diabetes. By definition, diabetes mellitus is categorized as a metabolic disease characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. The vast majority of cases of diabetes fall into two broad etiopathogenetic categories. The first category, type 1 diabetes, the cause is an absolute deficiency of insulin secretion. While the second type is much more prevalent category, called as type 2 diabetes, the cause is a combination of resistance to insulin action and an inadequate compensatory insulin-secretory response. Despite the great interest in the development of new drugs to prevent the burden of complications associated with this disease and the raised interest in the scientific community to evaluate either raw or isolated natural products in experimental studies, few of them were tested in humans. This chapter is a contribution to the understanding of ethnopharmacology of plants having hypoglycemic activity and its contribution to the elaboration of new treatment of diabetes mellitus.